Mated molded parts assembly and method for making same

ABSTRACT

A assembly of mated molded parts and a method for making same are disclosed in which the assembly has first and second parts with precisely mating surfaces. The first part is formed by molding, casting, or the like such that the mating surface of the first part is formed with at least one protrusion. The second part is formed in place against the first part such precisely mating surfaces are formed with the second part enveloping the protrusion. The two parts thus formed are forced apart from each other. The protrusion may be formed with a head which interlocks with the second part. When the parts are separated, the head breaks from the protrusion. In a particular embodiment, a bearing cap is disclosed having a central opening for the passage of a shaft.

BACKGROUND OF THE INVENTION Cross Reference to Related Application

This application is a continuation-in-part of application Ser. No.07/886,694, filed May 20, 1992, now U.S. Pat. No. 5,313,703.

FIELD OF THE INVENTION

The present invention relates to an assembly of mated parts and, moreparticularly, to an assembly of precisely interfitting and mating partsand to a method for making such an assembly.

DESCRIPTION OF THE RELATED ART

It is frequently desired to provide an assembly of parts in which thereis a precise mating or interfitting between the parts. As an example,bearing caps are frequently formed as split rings having two partssurrounding the shaft or bearing and joined together along a diametricline. With such split rings, not only must the central bore fit closelyto the shaft or bearing, but the parts of the caps must fit preciselytogether at the mating surfaces of the parts of the caps. If the partsof the caps are manufactured separately and individually for subsequentmachining and assembly, it is difficult to maintain the tolerancesnecessary to provide a sufficiently precise fit.

The term "bearing cap" as used herein denotes a device used to surround,support, and guide a rotating or reciprocating shaft. A typical use isat a point of penetration of the shaft through a housing or enclosuresuch as at the passage of a camshaft or crankshaft through an engineblock. Another typical use is at the passage of a crankshaft through apiston connecting rod. The bearing cap may make direct contact with theshaft, or, as is more frequently the case, the bearing cap may surrounda bearing which, in turn, directly contacts the shaft. In either event,the bearing caps must be manufactured to close tolerances in order toprovide a close, uniform fit around the shaft or bearing.

It has been proposed in the prior art to manufacture a two part bearingcap from a unitary bar stock or a cast unitary metal ring. The two partsof the bearing cap are made by first defining a diametric fracture planeby forming weakened diametric notches or grooves on the outer surfacesof the ring on either side of the central opening of the ring. Force isapplied to the portions of the ring at opposite sides of the notches orgrooves sufficient to fracture the ring along the fracture plane andform two separate ring halves. The intended result of this method isthat the fractured diametric surfaces of the ring halves will mateprecisely when the ring halves are placed around a shaft or bearing andbrought back together.

In practice, however, the aforementioned prior art method hassignificant drawbacks. First, when the ring is fractured, the metal atthe fracture surface is frequently splintered or distorted. Because ofthis distortion, the fracture surfaces do not precisely mate togetherwhen the ring halves are brought together. Second, since the ring mustbe fractured across its entire diametric cross-sectional area, the forcerequired to produce the fracture is huge. Such force is concentrated atthe points where the fracturing device contacts the ring, thus causingthe ring to bend and lose its desired dimensions. Third, an embeddedsteel reinforcing insert may not be used since the internal boundariesbetween the steel insert and the surrounding cast metal will definestress lines. The ring will tend to fracture along these stress linesrather than along the intended diametric plane.

In another prior art method of manufacturing a two part bearing cap, twosemicylindrical halves are first formed separately. The mating diametricsurfaces of each half are machined and bores are drilled into thesurfaces. Dowels are inserted into the bores and the two-halves areassembled together aligned by the dowels. Next, the central opening ofthe assembled bearing cap is machined to the desired diameter andfinish. The halves are then disassembled.

This second prior art method also presents significant drawbacks. Themethod is quite costly, requiring dowels, a step of machining the fourmating surfaces, and a step of drilling the four bores for the dowels.Several close tolerances must be held, including the positioning anddiameter of the dowel bores and the flatness and orientation of themating surfaces. A step of assembling bearing cap parts is requiredbefore the central opening can be machined, and it may be necessary tobolt the bearing cap parts together before the machining can be done.

The problems noted above are not limited to bearing caps or toassemblies having central openings. Such problems may arise wheneverseparable and assembleable parts are required to mate precisely.

Accordingly, there is a heretofore unmet need for an assembly ofprecisely interfitting parts an economical method for making such parts.

SUMMARY OF THE INVENTION

The present invention satisfies the aforementioned need by providing anassembly of molded parts in which a first part is molded separately anda second part is subsequently molded to interfit and interlock with thefirst part. When moldable material, such as molten metal, is moldedagainst the surface of a previously molded part, the material does notadhere to the previously molded part. The two parts are then separatedby pulling them apart. If the parts are made with an interlock,separation is accomplished by fracturing at the interlock or interfacebetween the parts. Since the second part is molded in place with respectto the first part, the mating surfaces of the two parts interfitprecisely and exactly together.

In a bearing cap made according to a preferred embodiment of theinvention, the mating surfaces of the first bearing cap part is formedwith frustoconical protrusions having enlarged distal ends. When thesecond part is cast, the second part envelops these protrusions, thuscapturing the enlarged ends of the protrusions. The two parts are thusheld firmly together during any machining operations which may berequired. When force is applied to separate the two parts, theprotrusions fracture at the neck by which the enlarged ends areconnected to the protrusions. These necks thus provide fracture zoneshaving a greatly reduced cross-sectional area as compared to the area ofthe mating surfaces of the parts. Thus, a only a relatively small forceis needed to fracture the necks of the protrusions and separate theparts. The small force required will not bend the parts.

When the parts of the bearing cap according to a preferred embodiment ofthe invention are first separated, recesses are formed in the secondpart which mate precisely with the protrusions on the first part. Thus,the protrusions and recesses provide a dowel-like means for preciselyaligning the two parts with respect to each other when the parts arereassembled.

According to an alternate embodiment of the invention, the protrusionsof the first part are not formed with enlarged ends or heads. Thisapproach is suitable for assemblies which do not need to be held firmlytogether prior to final assembly or for materials which shrink whenhardening.

In a preferred embodiment of the method of the invention, the parts ofthe assembly are formed in a die or mold having two cavities. The firstcavity is for forming the first part only. The second cavity is forreceiving a previously cast first part and for forming a second part ininterlocking or interfitting relationship with a previously formed firstpart. Thus, each cycle of the die can produce a separate first part anda fully formed assembly. If machining of the assembly is required, themachining may be performed prior to separating the two parts.

These and other objects, advantages, and features of the presentinvention will be more fully understood and appreciated by reference tothe written specification and appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective, somewhat diagrammatic, view of a bearing capaccording to the principles of the invention;

FIG. 2 is a front elevational view of the separately formed upper halfof the bearing cap of FIG. 1;

FIG. 3 is a bottom view of the bearing cap upper half of FIG. 2;

FIGS. 4, 5, 6 and 7 are perspective, diagrammatic, views of a die halfused according to the method of the invention to manufacture the bearingcap of FIG. 1, illustrating steps in the method;

FIG. 8 is a front elevational view of the bearing cap with the secondpart in vertical section prior to fracturing the protrusions of thefirst part; and

FIG. 9 is an exploded front elevational view of the bearing cap with thesecond part in vertical section subsequent to fracturing the protrusionsof the first part;

FIG. 10 is a perspective, somewhat diagrammatic, view of an assembly ofmated, molded parts according to the invention; and

FIG. 11 is an exploded front elevational view of the assembly of FIG.10.

DESCRIPTION OF THE PREFERRED EMBODIMENT

By way of disclosing a preferred embodiment, and not by way oflimitation, there is shown in FIG. 1 an assembly of mated molded partscomprising a bearing cap 10 which includes in its general organization afirst part 12, or upper half, and a second part 14, or lower half.Together, the parts form a circular disk having a central circularopening 16 for the passage of a shaft, the retention of a bearing, orthe like. Other, noncircular shapes are possible within the scope of theinvention. For example, if the bearing cap is used at the end of apiston connecting rod, first part 12 may have an integral connecting rodportion extending therefrom. At either side of the central opening, thefirst and second parts 12, 14 are mated together along diametricallydisposed planes 18, 20. The parts are held together in assembledrelationship by bolts 22, screws, or other suitable fasteners whichextend through holes as more fully described below. The parts arepreferably formed of cast aluminum. A suitable material is 380 aluminum.

Details of the first part 12, or upper half, are shown in FIGS. 2 and 3.The first part 12 is in the form of a semicylinder, or half ring, havingan outer surface 24, an inner surface 26, and a pair of planar matingsurfaces 28 disposed at either side of the central opening through whichthe shaft will pass. The first part is formed with a pair of countersunkbores 29 through which fasteners will pass when the bearing cap isassembled.

An integral protrusion 30 extends from each of the mating surfaces 28disposed axially perpendicularly to the mating surfaces. Each protrusionincludes an outwardly narrowing frustoconical portion 32, an enlargedhead 34, and a neck portion 36 joining the frustoconical portion to thehead. The neck portions 36 form areas of reduced cross-sectional areaacross which the protrusions may be fractured to separate the heads 34from the frustoconical portions 32.

An apparatus and method for manufacturing the bearing cap of theinvention are shown in FIGS. 4-9. In FIG. 4, it may be seen that the diehalf 40 is formed with two cavities 42, 44 and a bifurcated passageway46 for introducing molten metal into the cavities. As is well known inthe art, an identical, mating die half is used with the die half shownin the Figures.

Cavity 42 is used to separately cast the first parts 12 of the bearingcap. As such, cavity is generally semicylindrical with two smallerrecess 48 corresponding to the protrusions 30 shown in FIG. 2. Cavity 44is cylindrical, and conforms to the overall shape of the fully assembledbearing cap as shown in FIG. 1.

As shown in FIG. 4, cores 50 are positioned in the cavity 42 prior tocasting the first parts 12. As is well known in the art, these coreswill prevent metal from filling the countersunk bores 29 (FIG. 2). Thecores are removed after the bearing cap is fully cast.

Two die halves are brought together, and molten metal is introduced intothe cavity 42. As shown in FIG. 5, a first part 12 is thus formed. Next,as shown in FIG. 6, the newly formed first part 12 is removed from thecavity 42. The first part 12 is inverted and placed in the lower portionof cavity 44. New cores 50 are placed in the cavity 42. Similarly, cores52 are placed in the upper portion of the cavity 44 in alignment withthe countersunk bores formed in the first part 12. The protrusions 30 ofthe first part 12 extend upwardly into the unoccupied upper portion ofthe cavity 44.

Next, the two die halves are again brought together, and molten metal isintroduced into both cavities 42 and 44. As shown in FIG. 7, thisresults in the formation of a second bearing cap first part 12' in thecavity 42, and the formation of a bearing cap second part 14 in theupper portion of the cavity 44. The second part 14 is thus perfectlymated to the first part 12. The newly cast metal of the second part 14does not adhere to the previously cast metal of the first part 12 whichallows the two parts to be subsequently separated.

The bearing cap 10 thus formed is then removed from the die 40. Thecores 50, 52 are removed. Bearing cap 12' is transferred to the cavity44. The steps illustrated in FIGS. 6 and 7 are repeated, with each cycleproducing a fully formed bearing cap 10 and a separate first part 12.

As shown in FIG. 8, the metal of the second part 14 of a newly formedbearing cap 10 completely envelops the protrusions 30 of the firstpart 1. The heads 34 of the protrusions are captured by, and interlockedwith, the second part. At this point, the central opening 16 of thebearing cap may be machined to the desired diameter and surface finish.If necessary, the countersunk bores such as bore 29 may be drilled tothe correct diameter or tapped. The parts are rigidly held togetherduring such machining by the protrusions and heads interlocked with thesecond part.

To separate the two parts, a device having fingers 58 is inserted intothe central opening 16 of the bearing cap. The fingers are thenforcefully spread apart. As shown in FIG. 9, the spreading apart of thefingers 58 causes the protrusions 30 to fracture and break at their neckportions 36 thus separating the bearing cap parts 12, 14. Only arelatively small force is required to fracture the necks because oftheir small cross-sectional area. The heads 34 remain embedded in thesecond part 14.

The second part 14 thus formed is provided with planar mating surfaces60, inner surface 62, and outer surface 64 which mate perfectly with thecorresponding surfaces of the first part 12. A frustoconical recess 66is formed through each of the mating surfaces 60. Each recess 66 matesperfectly with the frustoconical portions 32 of the protrusions 30 ofthe first part 12. Thus, when the bearing cap is reassembled around ashaft or bearing, the frustoconical portions 32 and the recesses 66guide the two parts together into perfect coalignment.

The invention is not limited to casting or the use of metal. Otherforming techniques and other materials may be used. For example,assemblies according to the invention may be made using molded powderedmetal or molded plastic. It is sufficient that the materials used bemoldable to the extent that the material is initially flowable, formedinto the desired shape, then hardened. The terms "molded" and "moldable"as used herein are intended to encompass all such forming techniques andmaterials.

The invention is not limited to bearing caps and the manufacture ofbearing caps. FIGS. 10 and 11 diagrammatically show an assembly 70 afirst part 72 and a second part 74 which precisely fit together alongconfronting surfaces 76, 78. First part 72 is molded in the mannerdescribed above. First part 72 is molded with wedge-like protrusions 80.Second part 74 is molded against the first part 72 such that theprotrusions 80 form the recesses 82 in the second part.

In contrast to the protrusions 30 described above, protrusions 80 arewedge shaped rather than frustoconical. Furthermore, protrusions 80 arenot formed with enlarged heads. The enlarged heads may be omitted if itis not necessary to hold the parts together firmly prior to separationand reassembly. Also, the material of the second part 74 may shrink asit hardens such that the protrusions 80 are grasped within the recesses82. The parts may be separated by pulling them apart such that theprotrusions 80 are withdrawn from the recesses 82.

The assembly of parts according to the invention may be formed with onlya single protrusion and recess, or with more than two. The matingsurfaces of the parts, such as surfaces 76 and 78 in FIG. 11, need notbe flat or planar. The assemblies shown in the figures are butillustrative of an unlimited variety of types, shapes, andconfigurations of assemblies which may be made according to theprinciples of the invention.

The above description is that of a preferred embodiment of theinvention. Various alterations and changes can be made without departingfrom the spirit and broader aspects of the invention as set forth in theappended claims, which are to be interpreted in accordance with theprinciples of patent law, including the Doctrine of Equivalents.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. An assembly of matedmolded parts comprising:a first part having first and second matingsurfaces, each of said mating surfaces having at least one protrusionextending therefrom and terminating in a head, said protrusions beingfracturable; a second part having first and second surfaces, each ofsaid mating surfaces of said first part mating with said first andsecond mating surfaces, respectively, of said second part, said firstand second surfaces on each of said first and second parts beingdisposed on different sides of a void; each of said mating surfaces ofsaid second part having a recess enveloping said protrusion andinterlocking with said head, whereby said first part may be separatedfrom said second part by readily breaking said protrusion creating breaklines, and further whereby said first and second parts may be preciselyrealigned following breaking along the break lines.
 2. The assembly ofclaim 27 wherein said head is initially joined to said at least oneprotrusion by a neck portion providing a breaking zone of reducedcross-sectional area.
 3. The assembly of claim 2 wherein each of saidprotrusions narrows outwardly from the mating surface from which itextends.
 4. The assembly of claim 3 further comprising a bore formed insaid first and second parts for the insertion of a fastener for holdingsaid first and second parts in assembled relationship.
 5. An assembly ofmated molded parts comprising:first and second parts each having matingsurfaces mated together on different sides of a void; each matingsurface of said first part integrally formed with a protrusion extendingoutwardly from said mating surface, each of said protrusions capable ofready fracture; said second part being molded in place with respect tosaid first part, whereby each mating surface of said second partinterfits precisely with the mating surface of said first part, wherebyeach of said protrusions is enveloped by said second part, whereby saidfirst and second parts can be separated and realigned by fracturing andrealigning said protrusions.
 6. The assembly of claim 5 wherein each ofsaid protrusions terminates in a head interlocking with said secondpart.
 7. The assembly of claim 6 wherein said head is joined to saidprotrusion at a neck of reduced cross sectional area, and whereby saidfirst and second parts may be separated by pulling said parts apartuntil said neck breaks.
 8. The assembly of claim 5 wherein saidprotrusion narrows outwardly from said mating surface of said firstpart.
 9. The assembly of claim 5 wherein said first and second parts areformed of cast metal.
 10. The assembly of claim 5 wherein said first andsecond parts are formed of molded plastic.
 11. The assembly of claim 5further comprising coaligned bores formed in said first and second partsfor the insertion of fasteners for holding said first and second partsin assembled relationship.